Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

When your speed exceeds light, how does your mass become 'imaginary'?

In the relativistic equation for mass:

Rest Mass
mass = ------------------
2 1/2
( 1 - ( V/c) )

You will note that for V greater than the speed of light, you end up taking
the square root of a negative number...what mathematicians call an imaginary
root. All this is saying physically, is that the mass of a body increases
to infinity asymptotically as V APPROACHES the speed of light. Notice that
just AT V = C, the denominator becomes Zero, and the observed mass becomes
infinite. For any continuous acceleration, you will have to first deal with
getting to this point, before you have to worry about stepping over it to get
imaginary mass!!!